The Messinian Salinity Crisis-Nov. 2005

As far as I have been able to discover, there are no straightforward, all-embracing, books on the geological history of Europe. Research being done is published in articles or websites, and is in different languages. The following is what I have been able to find out about a subject which has interested me for some time, using Google and the OU e-Library. I hope others will be able to contribute more, and perhaps build up a body of knowledge on various topics about the continent we live on.

CENOZOIC Era

In the early 1960s, seismic soundings in the Mediterranean found ubiquitous evidence of an acoustic reflector a few hundred metres below the sea floor and closely simulating its topography – christened the M layer. A decade later drillings in 3000m depth brought sediment cores of anhydrites (precipitated only from saline ground-water above 30° C) and stromatolites (organic fossils of algae mats in an intertidal environment) to the surface, and also gypsum. These sediments indicated that six million years ago the Mediterranean, which had much the same topography as today and had been deep sea for many millions of years, had been a series of brackish lakes and dessicated tidal flats, (Hsu, 1983). Research since then has built up the picture of one of the greatest evaporitic basins in Earth history, not only in shallow marine basins, but also widespread in the deeper Mediterranean.

CENOZOIC

Chronology was difficult as little could live in such saline conditions so dateable marine fossils were lacking, but 87Sr, ∂18O, and ∂13C isotopes, magnetostratigraphy, astrochronology, and stratigraphical studies where uplift and exhumation have exposed sequences, mainly in Italy and Spain, have helped to calibrate timing. The much- debated, but now mainly- agreed, outline sequence appears to be:

7.24/6.88 Ma - alternations of marine marls and sapropels in Sicilian strata reflecting gradual modification of water exchange with the Atlantic;

Also controversial was the cause of the isolation of the Mediterranean and the major sea-level fall – glacioeustatic climate change, tectonic movements, or a combination? The latter explanation is now widely accepted. There was a global sea-level drop of ~60m in the early Messinian, with arid glacial maxima driven by changes in the Earth’s precession, of which there is evidence globally, but this would have been insufficient to close the marine gateways with the Atlantic. The north-western Black Sea experienced conditions similar to the Mediterranean but of lesser amplitude (Gillet et al., 2004).

Mediterranean evaporites, from Rouchy (2004)

The tectonic evolution of the western Mediterranean was complex and various models have been proposed. One, deduced from polar wander paths and stratigraphy, is that in the Oligocene the western Mediterranean was a back- arc basin in the convergence of Africa and Eurasia, and the extensional regime caused various terranes of the mountainous block between the Pyrenees and the Alps to split off and drift: south-east to become Calabria, Sardinia and Corsica, south to become the Algerian Kabylies, and south-west to become the Balearics and the Betic-Rif arc/Alboran block.

The Western Mediterranean at 10 Ma ago (from Rosenbaum et al. 2002)

Once the subduction zone of the latter reached the passive margins of Africa and Iberia it became eastdipping with slab-rollback towards the oceanic lithosphere of the Atlantic and closed the Tethyan marine corridor. For a while the waters of the Atlantic and Mediterranean mixed through the Betic and Rifean corridors (Figure 3), but by 6.1 Ma evidence of mammal exchange between Africa and Iberia indicate that a landbridge of sorts existed (Garcés et al., 1998). The sinking of the subducted slab caused uplift of the order of 1 km along the African and Iberian margins, which closed the Betic and Rif corridors, isolating the Mediterranean and triggering the Messinian Salinity Crisis.

This model is supported by extensive laser 40Ar/39Ar age dating combined with major and trace element, and O-Sr- Nd-Pb isotope studies of igneous rocks from southern Spain, the Alboran Sea and northern Morocco (Duggen, 2004).

Another feature of dessication of the Mediterranean is that at lowstand all its rivers incised deep canyons. The building of the Aswan Dam in the 1950s, 1200km from the sea, was complicated by the discovery of a deep narrow gorge in granite hundreds of metres below sea level, and found to be 2500m deep below the Nile at Cairo. The Rhone, Ebro and the Po also have gorges 2000m below sea level, filled with late Miocene gravels and early Pliocene marine fossils. Other buried gorges and channels have been found in Israel and Syria and linked to the mouths of modern rivers all round the Mediterranean. Atlantic and Pacific canyons are linked to turbidity canyons on the continental shelf, whereas the Mediterranean ones all appear to be drowned river valleys (Hsu, 1983).

The start of the Pliocene was warmer, and rainfall increased in drainage areas, but the sudden return to full marine conditions with deep- sea fossils, cannot be explained by a climatic change. The maximum sea-level rise was found to have been reached 170 Ka before re- flooding. Faulting or tectonic slumping opening the Gibraltar gateway in the Betic-Rif mountain arc have been suggested, but have not been supported by any study.

Researchers at the University of Rennes have recently put forward an intriguing suggestion that the MSC caused its own end by intense regressive erosion. They suggest that the Betic-Rif Cordillera at Gibraltar was relatively low-lying, and indeed may have periodically been washed over by Atlantic waters, together with what remained of the Betic and Rifean corridors, supplying the marine salts necessary for the formation of evaporites. They modelled the morphological evolution of the Straits of Gibraltar, and found that an east-flowing stream could over time have become deeply incised, so that a spillway turned into a permanent gateway. Seismic evidence in the western Alboran sea has revealed E-W directed canyons in the eastward-sloping downslope continuation of the Strait. (Loget et al., 2005.) There are estimates that refilling would have taken 100 years, with a flow 100 times bigger than Victoria Falls over a kilometre size drop (West, 2002). By the early Pliocene all the indications are that the Mediterranean had resumed deep marine conditions, with plentiful microfossils.

Much still remains to be discovered about the Messinian evaporitic ‘sandwich’ between the deep marine layers. The Lower evaporites are only known from seismic soundings and exposed strata where uplift has occurred. Funding is being sought for oceanic drilling deep enough to penetrate the layer of salt up to 2 km thick in places. The debate about timing in different basins keeps a substantial ‘Messinian community’ with its own website occupied. The chronological outline, and of how it started and finished, are substantially agreed however. Interest in different areas of the topic seems to divide on national lines: Italians dominate stratigraphical studies, helped by Spaniards, Moroccans, Turks, Rumanians etc; the French focus on fluvial canyons, and the north Europeans on tectonics.

The legacy of the salinity crisis is exploitable chemicals: in Sicily halite, potash salts and gypsum have been exploited commercially for centuries, and there are saltworks on all shores. The Mediterranean is still saltier than other seas, and is believed to sequester 5% of oceanic salt in its M-layer. And if the Gibraltar gateway had not opened, the Sea could not have become the hospitable base for ancient civilisations in the late Holocene.

GILLET H, LERICOLAIS G, REHAULT J-P, & DINU C (2004), Was the Messinian event recorded in the Black Sea? Abstract given at 4th International Congress on Environment & Identity in the Mediterranean: The Messinian Salinity Crisis Revisited, at Corte, Italy

HSU, K J (1983) When the Mediterranean was a Desert: a voyage of the Glomar Challenger, Princeton University Press. Now out of print but quoted extensively in Fuerbacher A.

ROSENBAUM G, LISTER G S & DUBOZ C (2002), Reconstruction of the tectonic evolution of the western Mediterranean since the Oligocene, Journal of the Virtual Explorer, 8, 107130.

ROUCHY J M (2004) What can be expected from coring the whole Messinian evaporitic succession in the deep basins of the Mediterranean? Presentation to 4th International Congress on Environment & Identity in the Mediterranean: The Messinian Salinity Crisis Revisited, at Corte, Italy,www.messinianonline.it